The paste() function in R is a nice way to concatenate sets of values together. Let’s say you need to make up some fake gene expression data to illustrate a heat map.
# use rnorm() to generate 100 random values to fill a matrix
gdat <- matrix(rnorm(100), nrow=10, ncol=10)
We use the rnorm() function to grab 100 random values from a Normal distribution, and place them into a matrix. If this matrix is supposed to represent gene expression values, typically the rows would represent genes, and the columns would represent samples or conditions. We can use the paste() function to create names for the rows and the columns.
# create 10 fake gene names to label the rows
rownames(gdat) <- paste("g", 1:10, sep="")
# create 10 fake sample names to label the columns
colnames(gdat) <- paste("s", 1:10, sep="")
# now we can draw a heat map that will have row and column labels
The paste function takes a series of arguments to be concatenated, as well as a separator for each concatenation event. This is good for many things such as creating file names from other values:
mutants <- c("sir1", "cen3", "rad51")
datafiles <- paste(mutants, "txt", sep=".")
Or for dynamically creating a plot label:
paste("number of genes detected:", NumberOfGenes)
The default separator for paste() is a space character. So in the example above, I left out the "sep" argument because a space character will be inserted by default. Whereas in the first example with the heat map I did not want spaces to occur in my fake gene and sample names, so I specified: sep="" to overide the default.
Once you start using paste(), you'll use it a lot, and more often than not I would say you want to concatenate things without a space, or any other character. So I often find myself typing: paste(a,b,sep=""). A short cut for this is a function called paste0()! The paste0() function is paste with no default seperator! I always forget about it, but if you can remember it will save you a little bit of typing and make your code more readable.
# create 10 fake gene and sample names